I know with bait cast fishing you want to re-spool at least once a year more if you fish regularly and if your fishing line is exposed to a lot of UV rays. I know from the eyes on the rod, dirt, oils, and salts from your hands will cause some break down and wear on the line, among other variables.

You should replace your fly line when you notice a drop in performance. If it is a floater, it should stay floating. If the line doesn't shoot as well, even after a good cleaning, you should replace it. I put over 100 fishing days on a Rio Grand over a two year period. It performed okay during its last days. When I put a new line on the rod, the rod cast like a new rod.